This invention relates generally to equipment for diagnostic and therapeutic radiology and methods of making the same and, more particularly, to liquid metal grooved bearings for x-ray tubes used in x-ray generating equipment, such as computerized axial tomography (C.A.T.) scanners and more particularly concerns a means for capturing debris, such as from liquid metal oxidation, mechanical wear, chemical corrosion etc., which can clog the bearing and diminish bearing performance.
Modern diagnostic x-ray procedures often require a rapid sequence of high energy exposures. These procedures rapidly increase the temperature of both the anode and the bearing assembly. When the tolerance temperatures of these systems are approached or exceeded, the x-ray tube may have its useful life shortened or may even fail outright. One way to limit anode temperature is to increase the size and/or the rotation speed of the anode. However, increasing the size or rotation speed of the anode increases the axial and radial loads on the bearing assembly, thereby reducing the life of the bearing.
X-ray tubes with rotary anodes require a bearing system to provide axial and radial support to the rotating anode during operations. Most x-ray tubes are produced with the bearing system comprised of two ball bearings having a soft metal coating as a lubricant. Another type of x-ray tube supports its rotor axially with a magnetic bearing and radially with a sleeve bearing using a layer of liquid metal as the lubricant. At least one of supporting surfaces of the sleeve bearing has helical grooves to improve retention of the liquid metal during anode rotation. The liquid metal lubricants used in these sleeve bearings are low melting point, low vapor pressure gallium alloys which wet and separate the opposing bearing faces without substantially attacking the bearing faces.
Yet another type of x-ray tube supports the rotor in both the axial and radial direction with sleeve bearings having helical grooves in a liquid metal lubricant. One other type of x-ray tube uses a magnetic bearing as the primary support mechanism for the rotor and a ball bearing system as a back-up support system for when the magnetic bearing is interrupted or switched off. This tube also uses a spiral grooved bearing lubricated with metal liquid lubricant between the rotor and a cooling tube located at the end of the rotating shaft. This sliding bearing serves primarily for dissipating the heat produced in the anode and at the same time serves as an electrical contact for supplying the high voltage potential.
Another type of bearing system commonly used in x-ray tubes comprises two ball bearings. Ball bearings used in x-ray tubes are typically lubricated with soft, solid metal coatings because the more conventional ball bearing lubricants are not suitable for use in the high vacuum operating environment of the x-ray tube. The solid metal coating type of lubricant does not adequately dampen the chattering noise of the ball bearings and is not durable when used at continuously high speeds and temperatures.
A quieter, more durable alternative to metal coated ball bearing systems is the so-called hydrodynamic or fluid-film bearing. Hydrodynamic bearings used in x-ray tubes typically comprise a pair of cooperating bearing surfaces with a low vapor pressure liquid metal lubricant disposed in a gap between the surfaces. The lubricant wets the bearing surfaces so as to completely fill the gap without allowing any contact between the surfaces, even under loaded conditions. At least one of the bearing surfaces may be provided with spiral grooves to enhance dynamic stability and load capacity. Hydrodynamic sliding bearings will not produce bearing chatter and have the potential for long life. Such a bearing requires a low melting point, liquid metal lubricant which wets the supporting bearing faces so that the gap between the supporting faces is completely filled by the lubricant without allowing any contact between the bearing surfaces even under loaded conditions. Furthermore, the liquid metal lubricant of these bearings functions well at dissipating heat produced in the anode and at the same time can serve as a contact for supplying the necessary high voltage potential.
However, at high operating speeds, instability can develop in the liquid lubricant which breaks up the liquid metal layer in a spiral groove bearing and limits the speed range of the bearing. One difficulty with liquid metal spiral groove bearings is that debris can clog the bearing. The debris evolves from a number of sources including liquid metal oxidation, mechanical wear, and chemical corrosion. If Mo is used for the bearing material, a reactive layer is always formed between the liquid metal and Mo. This layer can act as a precipitant, which can clog the bearing. When the bearing is clogged by this debris, its performance is substantially impaired in that the load capacity is diminished and the bearing causes the tube to experience power losses due to viscous drag increase.
Accordingly, there is a need for a hydrodynamic grooved bearing which can rotate at high speeds without debris impairing tube performance by diminishing load capacity or causing power losses due to increased viscous drag.